The present invention relates to a method and apparatus for controlling dampening water supplied to a printing machine.
In a printing machine such as an offset printing machine it is usual to always wet a printing plate with water so as to prevent adhesion of printing ink to blank portions and to improve adhesion of ink to portions including characters, pictures, etc. To supply ink to the printing roll of an offset printing machine, a plurality of inking rollers are combined with the printing roll, so as to thoroughly knead the ink and then uniformly supply the kneaded ink to the printing roll. Accordingly, it is necessary to maintain a delicate balance between the ink and the moistening water in order to produce high quality printed matters. More particularly, if the supply of the moistening water is too large, the adhesion of the ink becomes poor thus producing blur printing, whereas when the supply of the ink is excessive the resulting printing becomes dirty.
Water having a suitable pH value has been used as the moistening water. It has also been the practice to incorporate into the moistening water such substances as aquaphilic high molecular substances as gum arabic carboxymethylcellulose; acids such as nitric acid, tannic acid, chromic acid, etc.; salts such as ammonium nitrate, zinc nitrate, ammonium phosphate, potassium dichromate, antimony dichromate, etc., alcohols and surface active agents.
Which one of the additives should be selected, how much the additives should be used and how to vary these parameters in accordance with the characteristics of the paper to be printed or other printing conditions have relied upon the experience and skill of the operator. When the moistening water prepared in this manner is used repeatedly, the alkaline concentration and the temperature of the moistening water increase gradually, with the result that such problems as contamination of the printing surface, decrease in the surface tension of the moistening water and emulsification of the printing ink occur, as is well known in the art. Accordingly, it is impossible to provide clear printing over a long interval so that it becomes necessary to renew the moistening water every 2 or several hours. Thus, all water contained in a tank for supplying the moistening water to the upper and lower portions of the printing machine must be renewed which is not only troublesome but also requires interruption of the printing operation. Further, the quality of the printed matter before and after renewal becomes variable. The balance between the supply of ink and the supply of the moistening water is established when the inking rollers and the moistening rollers rotate at prescribed speeds. The speeds of these rollers vary at the time of renewal of the moistening water and often reduce to zero. Assuming that the moistening water has a suitable composition, the balance between it and inking would be established only after a length of paper, 60 meters for example, has been printed, during which the quality of the printed matter would be impaired. Since such poor quality printing is experienced before and after the renewal of the moistening water, poor quality printed matter would be produced over a length of 100 m or more at each renewal of the moistening water. Such renewal is necessary when the composition of the moistening water changes, and in practice, it is almost impossible to measure the composition and the quantity of the moistening water for replenishing fresh water and reduced constituents. The mere addition of fresh water can not assure adequate composition thus rendering it difficult to restore the predetermined balance between the quantities of the ink and moistening water. For this reason, best result can be obtained only by completely discharging the moistening water and by charging newly prepared moistening water. This requires a long time, during which the printing operation must be stopped. As above described, renewal of the moistening water involves difficult problems. As above described, the characteristics of the moistening water is influenced by temperature and other operating conditions. Moreover, when the moistening water is used repeatedly, its composition is varied by the addition of mineral substances contained in paper, oil, surface treating agents as clay, and short fibers of paper. Any efficient technique that can control these factors has not yet been available and such adjustments are made by relying upon the experience and skill of the operator.